1. Field of the Invention
The present invention relates to an image forming apparatus for forming a plurality of images using image data recorded in a memory unit.
2. Description of the Related Art
In image forming apparatuses such as printers, digital copiers, facsimile machines and the like, the timing is individually controlled for image data input and operation of the printing unit to improve the printing productivity. For this reason, the input image data are temporarily stored in a page buffer memory, and subsequently read out with a suitable timing and output to the printer unit.
In color image forming apparatuses, yellow (Y), magenta (M), cyan (C), and black Bk color images or superimposed over another in order to form a full color image, and the image data of each color must be output to the printer unit with a suitable timing to prevent image drift of each color image. For this reason, the image data of each color are temporarily stored beforehand in a page buffer memory so as to control the readout and output with a suitable timing. In response to the demand to form image in a short time, some tandem-type color image forming apparatuses simultaneously store Y, M, C, Bk color image data in the page buffer memory beforehand, and read out the image data of each color with near simultaneity to form images in parallel.
When printing a plurality of copies, image data input from a scanner or printer controller are stored in a storage device such as a hard disk to eliminate re-input of the image data. Image data sequentially read out from this storage device in accordance with a print command are temporarily stored in a page buffer memory, and subsequently read out from the page buffer memory with a suitable timing and output to the printer unit.
The image data are compressed at a high compression ratio when stored on the hard disk to allow storage of many hundreds of pages. At the same time, this high compression ratio is achieved using a compression method that does not allow an editing process while in the compressed state (e.g., variable length compression method, JPEG compression method). The variable length compression method is discussed later. A description of the JPEG compression method is omitted since this method conforms to the ISO/IEC standard 10918. When image data are compressed at a low compression ratio, the capacity of the hard disk must be increased, with a resulting increase in cost. A further disadvantage is the long time necessary for writing to and reading from the hard disk.
The image editing process (e.g., rotation, N-in-1 for compressing N document sheets for output on a single sheet and the like) is conventionally performed on the image data stored in the page buffer memory. Therefore, the image data stored in the page buffer memory must be stored in an editable form. For this reason, the compressed image data must be expanded when transferred from the hard disk to the page buffer memory.
This expansion of the compressed image data requires a long time. Therefore, a long time is required for the transfer of the image data from the hard disk to the page buffer memory, and this process becomes a bottleneck which reduces the overall printing productivity.
An object of the present invention is to eliminate the previously described disadvantages.
A further object of the present invention is to provide an image forming apparatus of improved print productivity when printing a plurality of copies.
A still further object of the present invention is to provide an image forming apparatus capable of transferring data from a hard disk to a page buffer memory in a short time without reducing the compression ratio of the data written to the hard disk.
These and other objects are attained by an image forming apparatus having a buffer memory for storing image data, a printer unit for printing based on image data output from the buffer memory, a compression unit for compressing image data output from the buffer memory to generate compression data, a hard disk for storing the compression data compressed by the compression unit, an output means for outputting compression data from the hard disk to the buffer memory, and an expansion unit for expanding the compression data output from the buffer memory and outputting the expansion data to the printer unit.
The objects of the present invention are further attained by an image forming apparatus having a reception unit for receiving image data, a first compression unit for performing a first compression of received image data, an editing unit for editing based on the first compression data compressed by the first compression unit, a buffer memory for storing the edited first compression data, a first output unit for outputting to a printer unit the first compression data output from the buffer memory, a second compression unit for performing a second compression on the first compression data output from the buffer memory, a hard disk for storing the second compression data compressed by the second compression unit, a second output unit for outputting the second compression data from the hard disk to the buffer memory, and an expansion unit for expanding the second compression data output from the buffer memory and outputting the expanded data to the printer unit.
The invention itself, together with further objects and attendant advantages will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.